DE 100 59 173 C1 discloses a control interface for a three-phase ac motor via an inverter in secure technology. The power transistors of the inverter are controlled by means of means for inhibiting pulses. In the event of an error the respective power transistors of the inverter are blocked by means of the means for inhibiting pulses, by interrupting respective supply voltages for the control of the power transistors. The functionality of the means for inhibiting impulses is monitored regularly, e.g. after each time the supply voltage is powered on.